Electrospray ion (ESI) sources are used to produce gas phase analyte ions for analysis by analytical instruments, such as mass spectrometers. Under common ESI Mass Spectrometry (MS) conditions most analytes are not directly effected by the electrochemical process occurring while passing through the ESI source. Nonetheless, electrochemical reactions of analytes of interest can and do take place. These electrochemical reactions can alter the analyte molecules such that the ions observed in the gas phase have a different mass, charge, or both, than the original analyte molecule. In contrast, planned analyte electrochemistry can be very advantageous, providing the ability to create novel ionic species, probe analyte redox chemistry, and perform electrochemical ionization.
In general, problems with ESI source analyte electrochemistry arise where the analyte has a low oxidation or high reduction potential relative to the surface potential generated at the electrode surface in order to produce the current required for ionization. Several reports propose to eliminate this effect using homogeneous redox buffer solutions or sacrificial electrode materials to buffer the potential of the emitter to a degree where analyte electrochemistry does not take place. Unfortunately, both methods introduce products of the buffering reaction in the solution that may have unwanted effects. For example, the hydroquinone oxidation product benzoquinone can react with thiol moieties in an analyte resulting in an unintended mass shift in the mass spectrum, and oxidation of a sacrificial metal electrode introduces metal ions in the solution that may act as complexing agents thereby changing the characteristics of the mass spectrum.
An article by Van Berkel et al. discloses that it is possible to minimize electrochemical analyte reactions using controlled-potential systems. Gary J. Van Berkel and Kertesz, V., “Using the Electrochemistry of the Electrospray Ion Source,” Analytical Chemistry, p. 5510-5520 (Aug. 1, 2007). The Van Berkel article proposes to eliminate the undesirable electrochemical analyte reactions by adjusting the current supplied to the working electrode to prevent undesirable electrochemical reactions.